Gas Sensors Research Articles

Uniform Temperature Control for MEMS Gas Sensors Under Mismatch Conditions

Uniform Temperature Control for MEMS Gas Sensors Under Mismatch Conditions

Charge-Transfer Complexes: Fundamentals and Advances in Catalysis, Sensing, and Optoelectronic Applications.

Supramolecular assemblies, formed through electronic charge transfer between two or more entities, represent a rich class of compounds dubbed as charge-transfer complexes (CTCs). Their distinctive formation pathway, rooted in charge-transfer processes at the interface of CTC-forming components, results in the delocalization of electronic charge along molecular stacks, rendering CTCs intrinsic molecular conductors. Since the discovery of CTCs, intensive research has explored their unique properties including magnetism, conductivity, and superconductivity. Their more recently recognized semiconducting functionality has inspired recent developments in applications requiring organic semiconductors. In this context, CTCs offer a tuneable energy gap, unique charge-transport properties, tailorable physicochemical interactions, photoresponsiveness, and the potential for scalable manufacturing. Here, an updated viewpoint on CTCs is provided, presenting them as emerging organic semiconductors. To this end, their electronic and chemical properties alongside their synthesis methods are reviewed. The unique properties of CTCs that benefit various related applications in the realms of organic optoelectronics, catalysts, and gas sensors are discussed. Insights for future developments and existing limitations are described.

Evolution of transition metal dichalcogenide film properties during chemical vapor deposition: from monolayer islands to nanowalls

Unique properties possessed by transition metal dichalcogenides (TMDs) attract much attention in terms of investigation of their formation and dependence of their characteristics on the production process parameters. Here, we investigate the formation of TMD films during chemical vapor deposition (CVD) in a mixture of thermally activated gaseous H2S and vaporized transition metals. Our observations of changes in morphology, Raman spectra, and photoluminescence (PL) properties in combination with in situ measurements of the electrical conductivity of the deposits formed at various precursor concentrations and CVD durations are evidence of existence of particular stages in the TMD material formation. Gradual transformation of PL spectra from trion to exciton type is detected for different stages of the material formation. The obtained results and proposed methods provide tailoring of TMD film characteristics necessary for particular applications like photodetectors, photocatalysts, and gas sensors.

Effect of Ce doping on MoO3 thin films for room temperature ammonia sensing application

In this work, we report the fabrication and gas sensing application of undoped, and Ce doped (1, 2, 3, 4, 5 wt%) MoO3 thin films via simple, effective, and low-cost nebulizer spray pyrolysis method. The crystal structure of the prepared thin films was found to be monoclinic by the prominent peaks observed at (001), (002) planes and the primary peak intensities increases from undoped to 3% Ce doped MoO3 thin film. The morphology of the samples was studied by FESEM, and the films have nanofibrous network embedded with nanorods spread over the surface of the nanofibers. The optical properties were characterised by the UV–vis spectroscopy and observed that the film’s energy band gap declines from 3.28 to 3.04 eV due to the Ce dopant which alters the energy levels of the conduction and valence bands of the host by oxygen defects. The defects were analysed by the PL spectroscopy, and it proved that the PL emission peaks arose due to the oxygen deficiencies. The 3% Ce doped MoO3 produced higher PL emission intensity indicated that the higher oxygen defects sites are the cause. The gas sensing responses were measured for the pristine and 1, 2, 3, 4 and 5 wt% Ce doped MoO3 gas sensors increase from 6.48 × 102 to 1.67 × 104 and found to be higher for the 3% Ce doped sensor to detect ammonia gas. The significant gas sensing property such as rise time and fall time were observed to be least for the 3% Ce doped MoO3 thin film was 54 and 5 s. This study revealed that 3% Ce doped MoO3 thin film could be an efficient prominent ammonia gas sensor at room temperature in future.

High Selectivity in NO2 Gas Sensing Applications using Polythiophene-MnO2 Composite Thin Films

Many harmful gasses are released during the burning of fossil fuels. Thus, it is essential to create gas sensors with high efficiency and low power consumption. The easy chemical bath deposition approach was used to produce the MnO2-incorporated polythiophene (PTh/MnO2) composite thin film, and its potential application in NO2 gas sensing at the operating temperature was comprehensively examined. The physicochemical properties were examined using FT-IR, FE-SEM, EDAX, AFM, and other instruments. The effects of adding MnO2 on the physicochemical and NO2-sensing properties were studied. Out of all the oxidizing and reducing gases, the PTh/MnO2 composite sensor showed the highest NO2 selectivity when tested at operating temperature (NH3, Cl2, H2S, NO2, and methane). Every PTh/MnO2 composite thin film demonstrated improved NO2 sensing capabilities, with a maximum sensitivity of 95.67 percent for 100 parts per million of gas and a response of 20.93 percent for 0.5 parts per million of gas. Furthermore, they all reacted remarkably well to decreased NO2 gas concentrations.

Phase-Engineered MoSe2/CeO2 Composites for Room-Temperature Gas Sensing with a Drastic Discrimination of NH3 and TEA Gases.

Detecting and distinguishing between hazardous gases with similar odors by using conventional sensor technology for safeguarding human health and ensuring food safety are significant challenges. Bulky, costly, and power-hungry devices, such as that used for gas chromatography-mass spectrometry (GC-MS), are widely employed for gas sensing. Using a single chemiresistive semiconductor or electric nose (e-nose) gas sensor to achieve this objective is difficult, mainly because of its selectivity issue. Thus, there is a need to develop new materials with tunable and versatile sensing characteristics. Phase engineering of two-dimensional materials to better utilize their physiochemical properties has attracted considerable attention. Here, we show that MoSe2 phase-transition/CeO2 composites can be effectively used to distinguish ammonia (NH3) and triethylamine (TEA) at room temperature. The phase transition of nanocomposite samples from semimetallic (1T) to semiconducting (2H) prepared at different synthesis temperatures is confirmed via X-ray photoelectron spectroscopy (XPS). A composite sensor in which the 2H phase of MoSe2 is predominant lacks discrimination capability and is less responsive to NH3 and TEA. An MoSe2/CeO2 composite sensor with a higher 1T phase content exhibits high selectivity for NH3, whereas one with a higher 2H phase content (2H > 1T) shows more selective behavior toward TEA. For example, for 50% relative humidity, the MoSe2/CeO2 sensor's signal changes from the baseline by 45% and 58% for 1 ppm of NH3 and TEA, respectively, indicating a low limit of detection (LOD) of 70 and 160 ppb, respectively. The composites' superior sensing characteristics are mainly attributed to their large specific surface area, their numerous active sites, presence of defects, and the n-n type heterojunction between MoSe2 and CeO2. The sensing mechanism is elucidated using Raman spectroscopy, XPS, and GC-MS results. Their phase-transition characteristics render MoSe2/CeO2 sensors promising for use in distributed, low-cost, and room-temperature sensor networks, and they offer new opportunities for the development of integrated advanced smart sensing technologies for environmental and healthcare.

Construction of Fully Integrated and Energy Self-Sufficient NO2 Gas Sensors Utilizing Zinc-Air Batteries.

Exploration of novel self-powered gas sensors free of external energy supply restrictions, such as light illumination and mechanical vibration, for flexible and wearable applications is in urgent need. Herein, this work constructs a flexible and self-powered NO2 gas sensor based on zinc-air batteries (ZABs) with the cathode of the ZABs also acting as the gas-sensitive layer. Furthermore, the SiO2 coating film, serving as a hydrophobic layer, increases the three-phase interfaces for the NO2 reduction reaction. The constructed sensors exhibit a high sensing response (0.3 V @ 5 ppm), an ultralow detection limit (61 ppb), a fast sensing process (129 and 103 s), and excellent selectivity. Moreover, the sensors also possess a wide working temperature range and a low working temperature tolerance (0.34 V at -15 °C). Simulations indicate that the hydrophobic surface at the cathode-hydrogel interface will accommodate more NO2 gas molecules at the reaction sites and prevent the influence of inner water evaporation and direct dissolution of NO2 in the electrolyte, which is beneficial to the enhanced gas sensing abilities. Finally, the self-powered sensing device is incorporated into a smart sensing system for practical applications. This work will pave a new insight into the construction of integrated and energy self-sufficient smart gas sensing systems.

Fabrication, structural, morphological, and mechanical behaviour of fly ash doped clay ceramics based CO2 gas sensor

Abstract Various attempts have been made to fabricate fly ash-doped clay composites via solid state reaction method. Additionally, to investigate the structural, mechanical, surface morphology, and CO2 gas sensing behavior, the fabricated clay composites were sintered at three different temperatures 1000, 1100, and 1200 °C (COF1, COF2, COF3) for 4 h. The green and sintered densities of the fabricated composites were found to be in the range of 2.17-2.13 g/cm3 and 1.38 to 1.30 g/cm3. Further, various characterization techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, universal testing machine (UTM) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDAX) were carried out. Moreover, to determine the compressive strength and Young’s modulus values, a universal testing machine (UTM) was used. The fracture toughness of the fabricated composites, COF2 and COF3 were found to be 7.84 MPa-m1/2 and 2.22 MPa-m1/2. The COF3 composite exhibited a sensing response, response time, recovery time of 3.39 at 1200 ppm CO2, 16.95 s and 18.05 s respectively. Consequently, this porous clay composite can be fabricated in a cost-effective and eco-friendly manner, hence beneficial for CO2 gas sensing applications.

Pd Nanoclusters-Sensitized MIL-125/TiO2 Nanochannel Arrays for Sensitive and Humidity-Resistant Formaldehyde Detection at Room Temperature.

Among the various hazardous substances, formaldehyde (HCHO), produced worldwide from wood furniture, dyeing auxiliaries, or as a preservative in consumer products, is harmful to human health. In this study, a sensitive room-temperature HCHO sensor, MTiNCs/Pd, has been developed by integrating Pd nanoclusters (PdNCs) into mesoporous MIL-125(Ti)-decorated TiO2 nanochannel arrays (TiNCs). Thanks to the enrichment effect of the mesoporous structure of MIL-125 and the large surface area offered by TiNCs, the resulting gas sensor accesses significantly enhanced HCHO adsorption capacity. The sufficient energetic active defects formed on PdNCs further allow an electron-extracting effect, thus effectively separating the photogenerated electrons and holes at the interface. The resulting HCHO sensor exhibits a short response/recovery time (37 s/12 s) and excellent sensitivity with a low limit of detection (4.51 ppb) under ultraviolet (UV) irradiation. More importantly, the cyclic redox reactions of Pdδ+ in PdNCs facilitated the regeneration of O2-(ads), thus ensuring a stable and excellent gas sensing performance even under a high-humidity environment. As a proof-of-principle of this design, a wearable gas sensing band is developed for the real-time and on-site detection of HCHO in cigarette smoke, with the potential as an independent device for environmental monitoring and other smart sensing systems.

Impact of ternary nanocomposite In2O3/SnO2/F-CDs composition on low ppm NO2 sensing performance

Nitrogen dioxide (NO2) is a significant air pollutant with a notable environmental impact, necessitating precise monitoring. However, most existing NO2 sensing materials exhibit low sensitivity and require high-temperature operation. In this study, we employed a hydrothermal and co-precipitation method to synthesize a novel ternary nanocomposite, In2O3/SnO2 nanoparticles (NPs)/fluorine-doped carbon quantum dots (F-CDs). By varying the mass ratio of In2O3 to SnO2 NPs, we investigated the gas-sensing performance of the composite towards NO2. The results demonstrate that when the ratio of SnO2 and In2O3 reaches to 1 : 2, the 2-In2O3/SnO2/F-CDs gas sensor exhibited the highest response, excellent selectivity, and remarkable stability under ultraviolet (UV) light irradiation at room temperature (∼25 °C). Furthermore, the response of the 2-In2O3/SnO2/F-CDs gas sensor displayed a strong linear relationship with the concentration of NO2 gas. At a NO2 gas concentration of 3 ppm, the sensitivity was 238, which is significantly higher than that of In2O3/SnO2 NPs and In2O3 sensors. The response time and recovery time were measured at 62 s and 33 s, respectively. Finally, based on experimental characterization results and band structure analysis, we confirmed the promising effects of ternary nanocomposite in enhancing gas sensing.

ZrO2-CeO2/WO3 heterostructure films prepared by magnetron sputtering for humidity-tolerant triethylamine sensing

Up to now, the development of gas sensors based on metal oxide semiconductors (MOSs) that can detect triethylamine (TEA) under high humidity conditions remains a significant challenge. In this study, we present the ZrO2-CeO2/WO3 (ZCW) heterostructure films for TEA detection under high relative humidity, which were prepared by co-sputtering ZrO2 and CeO2 onto the surface of a WO3 film using the radio frequency magnetron sputtering method. The gas sensors based on the ZCW heterostructure films exhibited outstanding TEA-sensing performance, including high response (18.56–20 ppm), rapid response/recovery rate (1/5 s), excellent selectivity, wide detection range (0.5–500 ppm), outstanding reproducibility and stability, and ideal anti-humidity ability. The excellent TEA-sensing properties of the ZCW heterostructure films should be mainly attributed to their unique surface microstructure, abundant oxygen vacancies, and the formation of heterostructures between ZrO2, CeO2, and WO3. Compared to the WO3 film, both the CeO2/WO3 and ZrO2-CeO2/WO3 films demonstrated highly-enhanced humidity-tolerant performance, which should result from the redox reactions between the Ce3+ and Ce4+ ions. This study provides a general method for preparing the MOSs-based TEA sensors that could work under high humidity condition.

High responsive Pd decorated low temperature α-MoO3 hydrogen gas sensor

The H2 gas adsorption properties, which are the most crucial concept for a gas sensor, of the 2-dimensional (2D) MoS2 are limited compared to its oxide form of 2D α-MoO3. On the other hand, it is straightforward to form high surface-to-volume ratio nanostructures with MoS2. In order to get the benefits of the large surface and better H2 adsorption properties for the H2 gas sensor, MoS2 film grown by the chemical vapor deposition (CVD) method was converted into MoO3 with a thermal oxidation process at 380 °C under oxygen-ambient conditions. The grown MoS2 film has indicated that it is formed from the coalesced MoS2 flakes. An ultra-high response of 3 × 106 at a relatively low temperature of 100 °C was achieved for as low as 800 ppm hydrogen concentration. The hydrogen gas sensing mechanism has been discussed in depth using the double injection model, similar to the electrochromic mechanism.

Investigation on palladium gate electrode-based SOI junctionless FET for hydrogen gas sensing

This study provides a comprehensive investigation on palladium (Pd) gate electrode-based silicon on insulator (SOI) junctionless field-effect transistor (JLFET) for hydrogen gas (H2) sensing (Pd–SOI-JLFET). The device has a gate dielectric stack consisting of silicon dioxide (SiO2) and hafnium dioxide (HfO2). An extensive analysis was conducted to detect and identify the presence of hydrogen gas by examining several electrical characteristics such as drain current (IDS), transconductance (gm), output conductance (gd), energy band diagram (E), gate-to-source capacitance (CGS), and surface potential (Φs). Furthermore, a comprehensive investigation was conducted to examine the impact of the presence of H2 gas and variations in temperature on important parameters associated with the short channel effects (SCEs) including off-state current (IOFF), on-state current (ION), subthreshold swing (SS) and threshold voltage (Vth). In addition, the sensitivity analysis of the off-state current (IOFF) by considering process variation effect has been done. Sensitivity is also calculated at various temperatures for the detection of hydrogen gas molecule. At the temperature of 300K, the sensitivity values were obtained as 1.50083, 3.21754, 27.71483, 152.39617 and 2052.8 for pressure values 10−14 Torr, 10−13 Torr, 10−12 Torr, 10−11 Torr and 10−10 Torr, respectively. This analysis provides a thorough examination of the performance and efficacy of the Pd–SOI-JLFET hydrogen gas sensor highlighting its potential for a wide range of hydrogen sensing applications.

GeP3 monolayer as a promising 2D sensing materials in detecting SO2, H2S, SOF2 and SO2F2

The detection of SF6 decomposition components using gas-sensitive sensors is significantly important for characterizing internal insulation failures and assessing the operational status of SF6 gas-insulated equipment. In this paper, the adsorption properties of GeP3 monolayers for SO2, H2S, SOF2 and SO2F2 gases were investigated based on density functional theory. Four gas adsorption systems were constructed, and the adsorption mechanisms and sensing characteristics of GeP3 monolayers on target gases were investigated by calculating parameters such as adsorption energy, charge transfer, density of states, and recovery time, along with their potential application as resistive gas sensors and field-effect transistor sensors. It is demonstrated that GeP3 monolayers were suitable for the detection of SO2, H2S, SOF2 and SO2F2 gases, all of which exhibited good chemisorption with adsorption energies of −1.36 eV, −0.78 eV, −1.82 eV and −2.91 eV, respectively. The adsorption of SO2 and H2S is found to cause a significant change in the conductivity of the GeP3 monolayers, and desorption is achieved at the optimal operating temperature in only 54.428 s and 10.686 s, respectively. Also the adsorption of SOF2 and SO2F2 can make the work function of the GeP3 monolayers significantly larger. Consequently, the GeP3 monolayers have the potential to be used as a resistive gas sensor for SO2 and H2S gases, or as a field effect transistor sensor for SOF2 and SO2F2 gases. This study provides theoretical guidance for the development of GeP3-based sensors for monitoring the insulation status and operational conditions of SF6 gas-insulated equipment.

Facile Fabrication of Lilac-Like Multiple Self-Supporting WO3 Nanoneedle Arrays with Cubic/Hexagonal Phase Junctions for Highly Sensitive Ethylene Glycol Gas Sensors.

Metal oxides with nanoarray structures have been demonstrated to be prospective materials for the design of gas sensors with high sensitivity. In this work, the WO3 nanoneedle array structures were synthesized by a one-step hydrothermal method and subsequent calcination. It was demonstrated that the calcination of the sample at 400 °C facilitated the construction of lilac-like multiple self-supporting WO3 arrays, with appropriate c/h-WO3 heterophase junction and highly oriented nanoneedles. Sensors with this structure exhibited the highest sensitivity (2305) to 100 ppm ethylene glycol at 160 °C and outstanding selectivity. The enhanced ethylene glycol gas sensing can be attributed to the abundant transport channels and active sites provided by this unique structure. In addition, the more oxygen adsorption caused by the heterophase junction and the aggregation of reaction medium induced by tip effect are both in favor of the improvement on the gas sensing performance.

Toward highly sensitive, selective, and stable palladium‐based MEMS gas sensors for hydrogen energy security

Toward highly sensitive, selective, and stable palladium‐based MEMS gas sensors for hydrogen energy security

A Machine Learning Approach for the Classification of Refrigerant Gases

Combining an Internet of Things-driven approach with machine learning algorithms holds great promise in discerning pure gases across various applications. Interconnecting gas sensors within a network allows for continuous monitoring and real-time environmental analysis, producing valuable data for machine learning models. Utilizing supervised learning algorithms, like random forests, enables the creation of accurate classification models that can effectively distinguish between different pure gases based on their distinct features, such as spectral signatures or sensor responses. This groundbreaking integration of the Internet of Things and Machine Learning fosters the development of robust, automated gas detection systems, ensuring high accuracy and minimal delay in recognizing pure gases. Consequently, it opens avenues for enhanced safety, efficiency, and environmental sustainability in numerous industrial and commercial scenarios.

Using metal oxide gas sensors to estimate the emission rates and locations of methane leaks in an industrial site: assessment with controlled methane releases

Abstract. Fugitive methane (CH4) emissions occur in the whole chain of oil and gas production, including from extraction, transportation, storage, and distribution. Such emissions are usually detected and quantified by conducting surveys as close as possible to the source location. However, these surveys are labour-intensive, are costly, and fail to not provide continuous emissions monitoring. The deployment of permanent sensor networks in the vicinity of industrial CH4 emitting facilities would overcome the limitations of surveys by providing accurate emission estimates, thanks to continuous sampling of emission plumes. Yet high-precision instruments are too costly to deploy in such networks. Low-cost sensors using a metal oxide semiconductor (MOS) are presented as a cheap alternative for such deployments due to their compact dimensions and to their sensitivity to CH4. In this study, we demonstrate the ability of two types of MOS sensors (TGS 2611-C00 and TGS 2611-E00) manufactured by Figaro® to reconstruct a CH4 signal, as measured by a high-precision reference gas analyser, during a 7 d controlled release campaign conducted by TotalEnergies® in autumn 2019 near Pau, France. We propose a baseline voltage correction linked to atmospheric CH4 background variations per instrument based on an iterative comparison of neighbouring observations, i.e. data points. Two CH4 mole fraction reconstruction models were compared: multilayer perceptron (MLP) and second-degree polynomial. Emission estimates were then computed using an inversion approach based on the adjoint of a Gaussian dispersion model. Despite obtaining emission estimates comparable with those obtained using high-precision instruments (average emission rate error of 25 % and average location error of 9.5 m), the application of these emission estimates is limited to adequate environmental conditions. Emission estimates are also influenced by model errors in the inversion process.

Molecularly specific detection towards trace nitrogen dioxide by utilizing Schottky-junction-based Gas Sensor

Trace NO2 detection is essential for the production and life, where the sensing strategy is appropriate for rapid detection but lacks molecular specificity. This investigation proposes a sensing mechanism dominated by surface-scattering to achieve the molecularly-specific detection. Two-dimensional Bi2O2Se is firstly fabricated into a Schottky-junction-based gas-sensor. Applied with an alternating excitation, the sensor simultaneously outputs multiple response signals (i.e., resistance, reactance, and the impedance angle). Their response times are shorter than 200 s at room temperature. In NO2 sensing, these responses present the detection limit in ppt range and the sensitivity is up to 16.8 %·ppb−1. This NO2 sensitivity presents orders of magnitude higher than those of the common gases within the exhaled breath. The impedance angle is involved in the principle component analysis together with the other two sensing signals. Twelve kinds of typical gases containing NO2 are acquired with molecular characteristics. The change in dipole moment of the target molecule adsorbed is demonstrated to correlate with the impedance angle via surface scattering. The proposed mechanism is confirmed to output ultra-sensitive sensing responses with the molecular characteristic.

Cobalt-Doped Ceria Sensitizer Effects on Metal Oxide Nanofibers: Heightened Surface Reactivity for High-Performing Chemiresistive Sensors.

Chemiresistive gas sensors based on semiconducting metal oxides typically rely on noble metal catalysts to enhance their sensitivity and selectivity. However, noble metal catalysts have several drawbacks for practical utilization, including their high cost, their propensity for spontaneous agglomeration, and poisoning effects with certain types of gases. As such, in the interest of commercializing the chemiresistive gas sensor technology, we propose an alternative design for a noble-metal-free sensing material through the case study of Co-doped ceria (Co-CeO2) catalysts embedded in a SnO2 matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co-CeO2 catalyst nanoparticles integrated with SnO2 nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co-CeO2@SnO2 NFs exhibited superior gas sensing characteristics toward isoprene (C5H8) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co-CeO2 catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO2 sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C5H8 (compared to pristine SnO2), exceptionally high selectivity, and a low detection limit of 100 ppb. The sensor also exhibited high stability for prolonged response-recovery cycles, attesting to the strong anchoring of Co-CeO2 catalysts in the SnO2 matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co-CeO2, demonstrate strong potential to completely replace noble metal catalysts, thereby advancing the development of the commercially viable chemiresistive gas sensors free from noble metals, capable of detecting target gases at sub-ppm levels.